This invention relates to a method for working the surface of grain-oriented silicon steel to affect the domain size so as to reduce core loss. More particularly, this invention relates to providing localized strains and defects on the surface of grain-oriented silicon steel by capacitive electrical discharge.
In the manufacture of grain-oriented silicon steel, it is known that the Goss secondary recrystallization texture, (110)[001], in terms of Miller's indices, results in improved magnetic properties, particularly permeability and core loss over nonoriented steels. The Goss texture refers to the body-centered cubic lattice comprising the grain or crystal being oriented in the cube-on-edge position. The texture or grain orientation of this type has a cube edge parallel to the rolling direction and in the plane of rolling, with the (110) plane being in the sheet plane. As is well known, steels having this orientation are characterized by a relatively high permeability in the rolling direction and a relatively low permeability in a direction at right angles thereto.
In the manufacture of grain-oriented silicon steel, typical steps include providing a melt having on the order of 2-4.5% silicon, casting the melt, hot rolling, cold rolling the steel to final gauge with an intermediate annealing when two or more cold rollings are used, decarburizing the steel, applying a refractory oxide base coating, such as a magnesium oxide coating, to the steel, and final texture annealing the steel at elevated temperatures in order to produce the desired secondary recrystallization and purification treatment to remove impurities, such as nitrogen and sulfur. The development of the cube-on-edge orientation is dependent upon the mechanism of secondary recrystallization wherein during recrystallization, secondary cube-on-edge oriented grains are preferentially grown at the expense of primary grains having a different and undesirable orientation. Grain-oriented silicon steel is typically used in electrical applications, such as power transformers, distribution transformers, generators, and the like. The domain structure and resistivity of the steel in electrical applications permits cyclic variation of the applied magnetic field with limited energy loss, which is termed "core loss". It is desirable, therefore, in steels of this type to reduce domain wall spacing and thereby the core loss, as described in Journal of Metals, Vol. 38, No. 1, January 1986, pp. 27-31.
It is known that domain size and thereby core loss values of grain-oriented silicon steels may be reduced if the steel is subjected to any of various practices to induce localized strains in the surface of the steel. Such practices may be generally referred to as "scribing" or "domain refining" and are performed after the final high temperature annealing operation.
If the steel is scribed after the decarburization anneal but prior to the final high temperature texture anneal, then the scribing generally controls the growth of the secondary grains to preclude formation of large grains and so tends to reduce the domain sizes. U.S. Pat. No. 3,990,923, issued Nov. 9, 1976, discloses methods wherein prior to the final high temperature annealing, a part of the surface is worked, such as by mechanical plastic working, local thermal treatment, or chemical treatment.
If the steel is scribed after final texture annealing, then there is induced a localized stress state in the texture annealed sheet so that the domain wall spacing is reduced. These disturbances typically are relatively narrow, straight lines, or scribes generally spaced at regular intervals. These scribe lines are typically transverse to the rolling direction and are typically applied to only one side of the steel.
There have been attempts to refine domain spacing and improve the magnetic properties of steel after final texture annealing by subjecting the steel sheet surface to an electrical discharge from a probe located above the surface of the sheet to create a line of surface ablation and stress. European patent application No. 137747A, published Apr. 17, 1985, discloses a method and apparatus including an electrical discharge probe adapted to be located above the surface of the grain-oriented sheet at a gap of up to 3 millimeters. A high voltage supply having a negative polarity on the order of 12 kilovolts is used to provide a voltage for discharge on the order of 3-10 kilovolts. Such high voltage was found necessary for the spark to traverse the air gap between the probe and the steel sheet and break down the insulating coating on the steel. The reference disclosed a circuit which included a capacitor for regulating the energy delivered to the sheet. Moving the probe above and across the sheet will produce a line of ablation spots. In the alternative, a continuous arc discharge could be produced so that a continuous line of ablation is formed. The discharge spots are disclosed in the alternative as being provided by a fixed power supply by use of a trigger mechanism to discharge the capacitor. See also U.S. Pat. No. 4,652,316, issued Mar. 24, 1987.
In the use of such grain-oriented silicon steels during fabrication incident to the production of transformers, for example, the steel is cut and subjected to various bending and shaping operations which produce stresses in the steel. In such instances, it is necessary and conventional for manufacturers to stress relief anneal the product to relieve such stresses. During stress relief annealing, it has been found that the beneficial effect on core loss resulting from some scribing techniques, such as thermal scribing, are lost.
In a copending application, Ser. No. 047,967 entitled "Electrical Discharge Scribing for Improving Core Loss of Grain-Oriented Silicon Steel", filed May 8, 1987, of the same Assignee, the inventors describe a method and apparatus for electrical discharge scribing by producing a plurality of electrical discharges between a contacting electrode traversing the steel surface and the steel surface. The apparatus requires higher voltages and higher currents in an intricate electrical circuit for scribing base coated and finish coated steels in a high speed process.
What is needed is a method and apparatus for reducing the core loss values over that which exist in grain-oriented steels which are only final texture annealed, have base or stress coating thereon, and are not scribed. It is desirable that a method be developed for scribing such steel wherein the scribe lines required to improve the core loss values of the steel may be applied in a uniform and efficient manne to result in uniform and reproduceably low core loss values. A low cost scribing practice should be compatible with the conventional steps and equipment for producing such grain-oriented steels, and, furthermore, such improvements in core loss values should, preferably, survive stress relief annealing which are incident to the fabrication of such steels into end products.